You can use the plastic
bags used in American stores to put groceries in, as long as
they let a lot of sunlight pass. (Dark-colored bags will not do.)
I recently tested a bag used for fruits and vegetables, nearly
transparent and available free at American grocery stores,
that works great. This is stamped "HDPE" for high-density
polyethylene on the bag (ordinary polyethylene melts too easily).
A block of wood is placed under the jar to help hold the heat in.
(Any insulator, such as a hot pad or rope or even sticks, will
also work.)
A friend of mine who is also a Physics Professor did not believe
I could actually boil water with the thing. So I showed him
that with this new "solar funnel cooker," I was able to boil
water in Utah in the middle of winter! I laid the funnel on
its side since it was winter and pointed a large funnel towards
the sun to the south. I also had to suspend the black cooking
vessel--rather than placing it on a wooden block. This allows
the weaker sun rays to strike the entire surface of the vessel.
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